The present invention relates to a laser driver, and more particularly to a circuit and method for maximizing available compliance voltage in a laser driver circuit.
In this specification, transistors are shown as bipolar junction transistors (NPN's, specifically). This is for convenience and illustration, it being understood that the circuits illustrated and described may also be implemented with metal-oxide silicon (MOS) or other equivalent devices.
The most common implementation of a laser driver includes a differential pair of transistors in parallel with a constant current source. One common arrangement of transistors in a laser driver is shown in FIG. 1. The laser driver of FIG. 1 includes a pair of differentially coupled and driven transistors 12 and 14, hereinafter referred to as “the predrive differential pair 15”, and also as a “first differential drive circuit”. The predrive differential pair 15 has a common node 13 where the emitters of the transistors 12 and 14 are connected in common to a current source 17a. The bases of the transistors 12 and 14 are the input of the predrive transistor pair 15. A connection to a supply voltage (+) for the predrive differential pair 15 is provided through resistive connections to the respective collectors of the transistors 12 and 14. The components of an amplified differential signal are provided from the predrive differential pair through an output including nodes 16a and 16b to which the collectors of the transistors 12 and 14 respectively are connected.
A differential laser modulation signal is input to the differential pair 15 through the bases of the transistors 12 and 14, amplified and output via nodes 16a and 16b. The predrive differential pair 15 may be either DC or AC coupled to a laser diode 20 through combinations of passive components such as transistors, resistors, capacitors, or inductors. For example FIG. 1 depicts a DC coupled arrangement where the nodes 16a and 16b forming the output of the differential pair 15 are connected to the bases of respective NPN transistors 18 and 19. Current is provided to the transistors 18 and 19 by current sources 17b and 17c, respectively. The NPN transistors comprise a high speed buffer circuit which amplifies the two components of the amplified predrive differential signal produced by the predrive differential pair 15, providing the amplified components to the bases of transistors 24 and 25. The transistors 24 and 25 form a “drive differential pair” 30 (also a “second differential drive circuit”); their bases form the input of the drive differential pair 30. The emitters of the drive differential pair 30 are connected in common to a current source 17d. The drive differential pair has a connection to the supply voltage through the collector of the transistor 24, which is connected to the supply voltage +, and through the laser diode 20, which is connected in series between an output node 31 in the collector of the transistor 25 and the supply voltage +. The laser diode 20 is biased on at a low level by a DC current bias source 32 connected to the cathode of the laser diode and to the output node 31. The output of the laser driver consists of light produced by the lasing action of the laser diode 20 when it is biased at or above its compliance voltage.
One of the important objectives of a laser driver is the maximization of “compliance voltage” for the laser diode being driven. In this regard, a compliance voltage is the minimum voltage required by a specific, electrically-actuated component to operate. For example, it is necessary to have a minimum voltage across the laser diode 20 of FIG. 1 for it to be forward biased and to lase. A representative value for this voltage is 1.6 volts down from the supply voltage +, presuming a minimum compliance voltage of 2.3 volts available from the circuit. This minimum magnitude of this voltage (the laser diode's compliance voltage) is dependent on the drive current running through the laser diode, and the higher the drive current, the higher the compliance voltage. In FIG. 1, the compliance voltage is measured between the supply voltage (+) and the cathode of the laser diode 20. Laser diode characteristics vary widely with process and, consequently, so does the compliance voltage. Therefore, it is desirable to provide a laser driver that can maximize the voltage available to the laser while not adversely limiting the operation of the laser driving circuit itself (i.e., running into the compliance voltages of the other components that make up the driver circuit). Without maximizing the available compliance voltage to the laser it is possible that the laser may not lase under all conditions (i.e., process, temp, supply voltage).
There are advantages and disadvantages to DC or AC coupling in the context of achieving a given compliance voltage for the laser diode 20. AC coupling facilitates the achievement of the compliance voltage by separating the voltage level on the laser diode from that on the output driver. However, AC coupling requires additional components that are typically external to the IC implementation of the laser driver. AC coupling also requires that a termination resistor be placed on the output node of the drive differential pair. The presence of this termination resistor results in current division between the resistor and the laser diode. To compensate for this current division complicated feedback circuitry based on the optical output of the laser must be employed to obtain the desired optical output levels. The current division also requires the dissipation of more power than what is minimally required to achieve the desired optical output levels from the laser diode 20. In many applications (e.g. parallel fiber) the external AC coupling components, optical feedback mechanisms, and excessive power dissipation are not tolerable due to the requirements for tight spacing, low cost, and low power consumption. DC coupling in such applications is therefore the desirable mode as it eliminates the external components and feedback loop while minimizing power dissipation.
It is frequently the case in DC-coupled laser drivers that a large supply voltage is required to achieve the compliance voltage on the laser diode. However, with increasing demand for reduced supply voltages to minimize power dissipation a new scheme is required to optimize the laser compliance voltage.
In the laser driver of FIG. 1, the common mode voltage of the predriver differential pair 15 is sometimes modified in an open-loop fashion to maintain the drive voltage on the drive differential pair at levels that will prevent forward biasing of the transistor collect-to-base junctions in the drive differential pair. A diagram of such an implementation is shown in FIG. 2.
In FIG. 2, the laser driver of FIG. 1 is modified by provision of a common mode setting current source 41 connected in common to the two collector resistors 44 and 45 of the predrive differential pair 15 at a supply node 47. The supply node 47 receives a supply voltage for the predrive differential pair connected through a voltage sampling circuit preferably comprising a parallel RC circuit connected to the supply node 47. The parallel RC circuit includes a common mode bias resistor 50 connected in parallel with a capacitor 51 to the voltage supply +. The current source 41 mimics the circuit path to the drive differential pair 30 and provides a current at a level sufficient to cause the common mode voltage of the predriver differential pair 15 to be set at an appropriate level to obtain the desired output compliance voltage. While providing advantage over the laser driver of FIG. 1, this approach does not present a highly stable compliance range over process, temperature, and supply voltage such that the compliance voltage of the laser diode 20 is maximized. Consequently, AC coupling is still frequently used with this approach.